Using the controversy over the MMR vaccine, I consider the reasons why non-experts should defer to experts, and I sketch a model for understanding cases where they fail to defer. I first suggest that an intuitively plausible model of the expert/non-expert relationship is complicated by shifting epistemic standards. One possible moderate response to this challenge, based on a more complex notion of non-experts' relationship with experts, seems unappealing as an account of the MMR controversy. A more radical suggestion is that non-experts might have a political reason to defer to experts, when not doing so will involve ‘epistemological free-riding’. I investigate the implications.